This invention relates to an optical system for imaging distortions in a moving reflective sheet, such as distortions in glass sheets caused during a tempering process.
When glass sheets are tempered or annealed, they are transported through a heated furnace where they are heated to a critical temperature at which internal stresses are relieved. Achieving the critical temperature in a uniform manner over many square meters is difficult without softening the glass. Typically the glass reaches a softening point at which the glass is in a taffy-like state. The glass is transported on heat-resistant ceramic rollers, and gravity causes the softened glass to distort. Typical distortion is caused by sag in the glass between the rollers. If the furnace temperature is too high, sag can be pronounced. The sag is often cyclical in nature. The period of high points versus low points is related to a number of phenomena, two of which are the distance between adjacent rollers and the concentricity of the rollers. As an out-of-round roller turns, it imparts an imprint of periodic high points in the glass relative to its circumference.
When a glass sheet is conveyed out of the furnace, quenched, finished and ultimately installed in a building, the distortions due to tempering are noticeably visible. Beyond some tolerable level, such distortions are considered unacceptable, and the finished windows are rejected. When this occurs, the fabricator must replace the glass or the complete window unit.
The distortions caused by sag between the rollers or by an out-of-round roller tend to be cyclic in nature and to produce a corrugation effect in the glass sheet. These corrugation distortions are commonly known as roller wave distortions. All tempered or annealed glass has some roller wave distortion. While the glass industry quality standards specify an allowable peak-to-valley roller wave measurement, it has been difficult to accurately or easily measure roller wave distortion on the factory floor. Physical measurements are generally limited to only a small sample, due to the time required to measure with a dial indicator gauge. These physical measurements tend to be operator dependent with poor repeatability and are prone to error. Such tests may be destructive, often scratching the surface of the glass as the dial indicator gauge is rolled over the surface.
More commonly than physical measurement, an operator visually makes on-line measurements with a zebra board.
A zebra board consists of a large white backboard with black stripes painted diagonally across the board. As the glass sheets exit the quench area of the furnace, an operator views an oblique reflection of the zebra pattern in the glass as it moves on the conveyor. Roller wave distortions cause the straight diagonal lines of the zebra board to appear wavy in reflection. The more pronounced the roller wave, the wavier the lines appear. Operators are trained to judge the quality of the glass sheets subjectively, based upon the reflected image.
While helpful and inexpensive, a zebra board has limited use as a quality control tool. Customers are demanding improved and quantifiable quality in tempered glass products. Operators vary in their quality judgments, and consistent quality control is therefore difficult to achieve. There is no way to consistently quantify and document the results. Operators with the tasks of labeling, sorting and unloading glass have limited time for roller wave inspection.
When a glass sheet is fabricated and installed in a building window, an automobile, or a mirror, local distortions attributable to tempering or annealing may be noticeably visible. The visibly distorted areas of the glass act as lenses, bending the light in reflection and transmission, causing the light rays to displace and distort either reflected or transmitted images. Optical distortion is considered unacceptable beyond tolerable thresholds.
The glass industry has typically limited measurement of distortion to small samples in two dimensions, depth (z) and length in the direction of travel (y). Using a simplifying assumption that distortion is primarily a function of the rollers in the heat-treatment equipment, the industry has limited measurements to address roll wave or corrugation in the glass. Roll wave is described as long troughs and peaks parallel to the ceramic transport rollers, and is cyclical in period. Although roll wave distortion is problematic, local distortion across the glass in directions perpendicular to the transport rollers (x) is also highly problematic. This distortion is sometimes referred to as pocket distortion and is described as local point peaks and point valleys. Other distortion is referred to as micro corrugation and is described as corrugation in the glass at various angles due to draw lines or pullers used to spread the glass. No practical means exist for measuring these various distortions on a production line.
Current measurement means are limited to measurement of very small samples of glass relative to the volume of glass manufactured on the lines. Three such measurement means are flat bottom gauges, zebra boards (as described above) and interferometers. A flat bottom gauge with a dial indicator is used to measure gross peak-to-valley corrugation. Measurements are limited to a small sample, due to the time required to extract a sample from the process and measure with a dial indicator depth gauge over the entire surface of the glass. The sample is placed on a precision flat surface. Instrumentation is operator dependent with poor repeatability and is prone to error. Such tests may be destructive, often scratching the surface of the glass as the dial indicator gauge is rolled over the surface. The dial indicator is not able to measure pocket distortion or micro corrugation due to lack of resolution.
Further, the dial indicator depth gauge measures only one of the two parameters necessary to correlate the physical distortion to the optical distortion. Optical lens power is measured in diopters. Lens power (diopters)=1/f, where f is the focal length of the glass in the local region. 1/f=1/2R where R is the radius of curvature of the glass in the local region. The dial indicator makes an incorrect simplifying assumption that local distortion in glass is of constant radius of curvature, R. In fact, the radius of curvature of a local distortion is entirely dependent on local conditions during cooling and is infinitely variable.
Finally, interferometric measurements involve removing a strip of glass approximately 1 meter long by the full width of the float line. The sample is cooled, cut into 100 mm square samples and measured using interference fringe patterns through an interferometer. The method is highly accurate but destructive, and impractical for measuring glass from a float or tempering line. Three persons are required to measure less than 0.1 percent of the glass produced.
None of the methods in use is able to measure pocket distortion. Pocket distortion is physically too shallow to be measured using a dial indicator depth gauge, to random to be measured using interferometry, and not able to be quantified using a zebra board.
Customers of glass products demand ever-improved optical quality. Operators vary in their quality judgments, and consistent quality control is difficult to achieve. There is no practical means to consistently measure, quantify and document distortion in large volumes of glass. There is no practical means to measure pocket distortion and micro corrugation on a production line in real time.
Several automatic measurement systems have been proposed to solve this problem. U.S. Pat. No. 4,585,343 discloses a measurement apparatus for detecting and measuring roller wave distortions in sheet glass. This patent teaches a method of measuring a reflected pattern and comparing it to a control image. The preferred embodiment uses a plurality of complex optical systems, each of which requires precise alignment. The patent teaches a light pattern of 1×5¾ inch geometry changes as it is reflected from a concave versus a convex surface. It assumes the distortion in the glass is limited to uniform corrugation transverse to the direction of travel. The method collects a small sample of information in two dimensions, Y and Z.
U.S. Pat. No. 4,647,197 discloses a measurement apparatus using a CCD camera to detect a zebra board pattern transmitted through a windscreen. The pattern from a sample is recorded and compared to a reference pattern to quantify the quality of the sample.
U.S. Pat. No. 5,251,010 discloses a device for measuring roller wave using two parallel beams of monochromatic light. The preferred embodiment is limited to measuring one narrow strip of glass from a tempering furnace. As with U.S. Pat. No. 4,585,343, the device is limited to measuring corrugation in Y and Z directions and relies on the simplifying assumption that the glass is uniformly corrugated transverse to the direction of travel.
U.S. Pat. No. 5,726,749 discloses an apparatus for measuring distortion in curved windscreens using the angular deviation in a plurality of collimated light sources and an equally plurality of receivers, all precisely aligned. The method requires transmission of light through the glass and is directed at measuring distortion due to wedge variation and curvature variation in a laminated automotive windscreen.
Accordingly, there is a need for an improved optical detection apparatus for imaging and measuring optical distortions in reflective sheets.